Scintillation materials have been used for the detection of radiation. Plastic scintillators are used to detect the presence of ionizing radiation in applications such as detection of illegal transport of radioactive and fissile material, monitoring and safeguarding nuclear stockpiles, service of nuclear-nonproliferation, operation of nuclear research and power reactors, monitoring the use of medical and industrial isotopes, and in high energy, cosmic, and nuclear basic research. Plastic scintillators have been found to be efficient in detecting fast neutrons. However, plastic scintillators, to date, have shown little value for applications involving the detection of fast or thermal neutrons in the presence of background gamma rays. The detection of neutrons is important because they are strongly indicative of the presence of fissile material, such as plutonium and enriched uranium.
Ionizing radiation energy deposited in a scintillator material is typically converted into light. This light can then be measured by photo-sensitive detectors. Generally, incident penetrating radiation includes high-energy particles such as neutrinos, weakly interacting particles of all kinds, and ionizing radiation such as x-rays, gamma rays, alpha and beta particles, and fast and thermal neutrons.
U.S. patent application Ser. No. 13/430,394, discloses a plastic material composition that permits detection of fast and thermal neutrons. U.S. patent application Ser. No. 13/430,394, is incorporated by reference in the present application in its entirety.
A recent review of neutron detection technologies is Kouzes R. T. and Ely J. H., 2010; “Status Summary of 3He and Neutron Detection Alternatives for Homeland Security” PNNL-19360, Pacific Northwest National Laboratory, Richland, Wash. Despite great effort having been expended by many researchers, there is no efficient, very large area, low cost neutron detection technology available. Organic liquid scintillators have been employed to detect fast and thermal neutrons. For these detectors, the discrimination against gamma rays is achieved with the use of pulse shape discrimination (PSD). In this method, the gamma ray and neutron induced scintillation pulses are distinguished by the unique temporal signal characteristics of the scintillation pulse. A recent review of progress in PSD in boron loaded liquid scintillators has demonstrated that the technique has become very powerful for small detectors using commercially available liquid scintillators; see Mark Flaska and Sara A. Pozzi, Nuclear Instruments and Methods in Physics Research, Vol. 599, Issue 2-3, 221-225 (2009). However, the method suffers from two significant disadvantages. The low scintillation light yield resulting from neutron capture by boron imposes a severe upper limit to the size of a detector. All current commercially available neutron sensitive liquid scintillators contain boron. Also, there are major safety concerns in using large volumes of toxic, flammable, liquids at port and border locations. For these two reasons, large area, liquid scintillators have not been deployed at such locations.
Existing Portal Monitors have used polyvinyltoluene (PVT) plastic scintillator to measure gamma ray flux with essentially no energy resolution. In addition, a small area (about 0.5 m2) sodium iodide detector has provided moderate energy resolution (10% at 660 keV) for the detection of gamma rays. Thus, there is a continuing need for a gamma ray detection system to reliably detect even lightly shielded Special Nuclear Material (SNM).
In summary, there is a need for a large area, robust, economical detector material and system that can detect both fast and thermal neutrons and also gamma rays with a moderate energy resolution. The use of a single material, coupled to a single photosensitive readout and associated electronics, will provide an economical solution to the problem of detecting Special Nuclear Material.
It is well known that the detection sensitivity to fissile material can be increased by exposing the shipping container to an external beam of gamma rays or neutrons. Known as active interrogation, this process creates a relatively high radiation exposure to the container and its surroundings. For this type of application, it is desirable that the detection system has a high count rate capability, very high gamma discrimination, and good resistance to damage from elevated radiation levels.
In summary, there is a continuing need to have a single, large area, cost effective, robust, bright scintillating material that offers methods and systems for detecting one or more of the following:
1) fast neutrons, with good discrimination against gamma rays;
2) gamma rays, with moderate energy resolution (<15% FWHM at 660 keV); and
3) thermal neutrons, with good discrimination against gamma rays.
In addition, these detection systems should exhibit high count rate ability, good radiation resistance, low-toxicity, low-flammability, and long-term stable operation.